This invention concerns a device for detecting the tuning frequency of a frequency-modulation radio receiver, and more especially a device for detecting the tuning frequency of a frequency-modulation radio receiver which is moving in relation to broadcasting stations transmitting on different frequencies.
Such a device may be used in a radio-link system between a train and fixed stations along the track. There may be three different transmitting frequencies, for example, and two consecutive fixed stations transmitting on different frequencies. When the receiver on the train covers the distance between two stations, it must at a certain moment receive an order to stop receiving the frequency of the station it has left, and to start receiving the frequency of the next station.
In conventional systems of this kind, the receiver comprises different local oscillators, supplying the different tuning frequencies corresponding to transmitting frequencies, and the correct local oscillator is chosen by means of a tuning-frequency detecting or searching device.
This tuning-frequency searching device usually operates on the principle of examining the quality of the demodulated signal.
In a known device of this kind, examining the quality of the demodulated signal is achieved by isolating the noise from the effective signal, then measuring the length of time during which the noise level is above a certain threshhold. If this period exceeds a certain threshhold value, the decision is taken to switch from one local oscillator to another.
This method consequently requires a band-pass filter to select the noise outside the effective signal band, a threshhold detector to select noise which exceeds a certain threshhold level, a clock generator combined with an "AND" gate circuit to measure the duration of the noise selected by the threshhold detector, and a counting system to measure this duration and compare it with the threshhold level.
The present invention concerns a simplified device for detecting a tuning frequency, in which a completely different method is used to examine the quality of the demodulated signal.
The invention makes use of a phenomenon usually regarded as a form of undesirable disturbance, known as "clicks," described in particular in Time series analysis by S. O. Rice (ed. M. Rosenblatt, Wiley 1963, pp. 395-422).
These clicks are noise pulses which appear in an asynchronous demodulator, beyond the demodulation threshhold, caused by a sudden squaring of the noise and the effective signal. Their amplitude, while theoretically infinite, is in fact far higher than that of the noise and effective signal, and their number is in inverse proportion to the ratio of signal to noise at medium or average frequency.
The demodulated signal to noise ratio can therefore be found by measuring the frequency of these clicks; the ratio of signal to noise at low frequency can be compared for different frequencies, simply by comparing the frequency of these clicks: the right tuning frequency is the one at which said frequency is lowest.